At any moment, the ratio between them is a measure of the time elapsed, as long as the system remains closed.
One line of evidence involves rocks from outside the Earth--meteorites and moon rocks.
Radiometric dating shows that almost all meteorites are between 4.5 and 4.7 billion years old.
Once scientists have determined the parent-daughter ratio, they can use this measurement along with half-life of the parent to calculate the age of a rock containing the radioactive isotope.
Radiometric dating has shown that very old rocks--3.5 billion years or older--occur on all the continents.
One way to think about the closed system of the crystal is to compare it to an hourglass.